1. Field of the Invention
The present invention relates to an image reading apparatus that reads an image on a document using line sensors.
2. Description of the Related Art
Attention has been paid to an image reading apparatus that reads a large-format document (for example, A0 size, B0 size). Japanese Patent Application Laid-Open No. 2006-109408 discusses an image reading apparatus that includes a reading unit in which a plurality of line image sensors is arranged in the staggered pattern along the document conveyance direction.
In many cases, a roller used for the document conveyance on such an image reading apparatus is one undivided conveyance roller. However, as the roller becomes longer, the document conveyance accuracy tends to vary due to variations in diameter, eccentricity, deflection of the roller axis caused by the self-weight of the roller, and torsion of the roller during document conveyance. Therefore, the positional deviations in the joint portions of a read-image become irregular and vary at a non-periodic interval. The larger the format is, that is, the longer the conveyance roller is, the more obvious the problem becomes.
In addition, the image reading apparatus discussed in Japanese Patent Application Laid-Open No. 2006-109408 makes adjustment to eliminate positional deviations in the joint portions of a read-image that are caused by relative deviations among the line image sensors arranged apart in the document conveyance direction. To perform this adjustment, an adjustment sheet, on which a dedicated pattern is formed, is prepared in advance. Before use, the image reading apparatus reads the pattern from the sheet to perform pre-adjustment. The user may feel it very cumbersome to perform such a pre-adjustment operation because it has nothing to do with a reading operation.